1. Field of the Invention
The present invention relates to methods of measuring pressure of hydraulic fluid within hydraulic drive devices, e.g., brake devices and power steering devices of vehicles. In particular, the present invention relates to methods of measuring a pressure of a fluid that is supplied from the hydraulic drive device to an operation device, e.g. wheel brakes. The present invention also relates to methods of evaluating the condition of the hydraulic fluid within the hydraulic drive device in terms of soundness. For example, such soundness may include entrance of air into a flow path(s) of the hydraulic drive device. Furthermore, the present invention relates to hydraulic drive devices for carrying out these methods.
2. Description of the Related Art
Brake devices are known that include a pump for pressuring a brake fluid, an accumulator for accumulating the pressure of the pressurized brake fluid, and a pressure control unit, e.g., a hydraulic booster, that receives a supply of the pressurized hydraulic fluid from the accumulator. The hydraulic booster includes a pressure control valve for adjusting the pressure of the pressurized hydraulic fluid supplied from the accumulator in response to a stepping force applied to a brake pedal by a driver of a vehicle. The pressurized hydraulic fluid having the adjusted pressure is then delivered to wheel brakes.
In recent years, multifunctional brake devices, e.g., automatic brake devices, have been proposed. In general, such multifunctional brake devices require measurement of the pressure of the brake fluid in order to perform necessary function(s). In addition, when air has entered the flow path of the brake device or when the brake fluid within the flow path has leaked from the flow path, e.g., a pipeline(s), such troubles may influence on the control force(s) applied to the wheel brakes. However, the recognition of such troubles has been relied upon change in operation feeling of the driver who steps on the brake pedal. Therefore, there has been a potential demand for enabling evaluation of soundness that may be caused by the troubles mentioned above.
However, in general, wheel brakes are located away from a pressure control unit (e.g., a hydraulic booster). Therefore, pipelines that connect the pressure control unit and the wheel brakes must have relatively long length(s). In addition, in recent years, brake devices that are equipped with electronic control systems, e.g., anti lock brake control systems (ABS), have been increased. The brake devices that are equipped with such electronic control systems generally have pressure regulators. The pressure regulator includes a plurality of solenoid valves in order to control or regulate the pressure of the brake fluid that is supplied to each wheel brake. Naturally, the solenoid valves have orifices that may produce resistances against the flow of the brake fluid. Therefore, in some cases, the pressure may not be rapidly transmitted from the pressure control unit to the wheel brakes. In other words, the transmission of pressure may be delayed. In particular, such delay of transmission of pressure tends to be caused when a driver of a vehicle has abruptly stepped on a brake pedal or when the environmental temperature is relatively low.
If the delay of transmission of pressure has been caused between the pressure control unit and the wheel brakes, the pressure of the brake fluid within the pipelines as well as the pressure within the pressure control unit and the pressure within the wheel brakes may not be uniformly distributed along the length of the pipelines. Therefore, in some cases, a pressure detected by a sensor disposed in a portion of the pipelines is different from a pressure of the brake fluid that is actually supplied to the wheel brakes. As a countermeasure for avoiding such discrepancy, the measurement of the pressure may be made after the pressure distribution has become uniform. However, this countermeasure requires to wait for a long time that is sufficient for stabilization of the pressure distribution, because no measure has been known in order to predict when the pressure distribution becomes uniform. As a result, the measurement of the actual pressure supplied to the wheel brakes has consumed much time.
Incidentally, methods are known that utilize the result of detection of the pressure of the brake fluid in order to evaluate soundness of the brake devices. For example, the detected pressure may be used for ensuring that no air has entered the brake fluid within the pipelines. However, there have been increased demands for improved evaluation methods that utilize a rational process in order to enable rapid and reliable evaluation.